Provenance and Tectonic Settings of Devonian Sedimentary Strata in Xichuan Area,Northern Margin of South Qinling Block

The Devonian sedimentary strata,widely developed in the Qinling orogenic belt,are good vehicles to reveal the ocean-continent transition,tectonic attributes and tectonic settings of the Qinling orogenic belt in the Middle Paleozoic.This study presents new geochemical and zircon U-Pb isotopic data for the Devonian sedimentary rocks in Xichuan area.Geochemical data show that the Devonian clastic rocks are characterized by moderate compositional maturity,and a higher portion of sediment recycling,and are likely to have been derived predominantly from felsic igneous rocks within upper-continental-crust.The detrital zircon ages are mainly concentrated in Late Neoproterozoic-Paleozoic(0.40-0.63 Ga),Neoproterozoic(0.7-0.9 Ga)and Mesoproterozoic(1.0-1.6 Ga),with a small amount of ages in Paleoproterozoic and Middle-Late Archean.Comprehensive analysis suggests that the Devonian basin in Xichuan area has beea formed in passive continental margin,and its provenance may be mainly the uplift basement and tectonic highland of the South Qinling,not from the North Qinling.The subduction accretion event along the Shangdan Ocean probably did not affect the interior of South Qinling.